Dennis O'Brien was selected by the Minnesota North Stars in the second round, 14th overall, in the 1969 NHL Amateur Draft. He played for the North Stars for almost his entire career from 1970–71 to 1977–78. During his final season with the North Stars, he played just 13 games before being traded to the Colorado Rockies. After 16 games with the Rockies, he was traded to the Cleveland Barons and after 23 games as a Baron, he was traded to his fourth team of the season, the Boston Bruins, where he played 16 games.

The final trade to the Bruins broke an NHL record for the most teams played for in one season. That record has since been equalled by Dave McLlwain in 1991–92. McLlwain is still playing in the DEL for the Kölner Haie.

After that busy season, O'Brien played only two more seasons in the NHL, both for the Boston Bruins.